The present invention relates to an electrophotographic apparatus capable of forming a high quality image by contact type development in which a photoconductive belt and a developing roller are held in contact.
Among conventional developing systems using a single component developer, i.e., a toner, a system of the type maintaining a blade in contact with a developing roller to charge and regulate the thickness of the toner is predominant. In the case of non-contact development wherein the developing roller does not contact a photoconductive element, it is necessary to apply an AC bias to the developing roller. This kind of development is generally referred to as a jumping development system. When the developing roller is operated in contact with a hard photoconductive element, the developing roller and photoconductive element make line-to-line contact since both of them are hard. The problem with the line-to-line contact is that the nip width of the developing roller and photoconductive element and, therefore, the contact thereof is extremely unstable. This is apt to result in a partly omitted image or similar defective image. Moreover, the linear velocity of the developing roller is usually three times to four times as high as that of the photoconductive element. Consequently, an extremely great torque is required to drive the developing roller and photoconductive element both of which are hard.
In the light of the above, it is a common practice with the contact type development to use a photoconductive element implemented as a belt. The belt is passed over and driven by two to three spaced rollers. The developing roller is held in contact with the belt at a particular position between the rollers over a predetermined nip width. The developing roller is rotated at a linear speed about three times as high as that of the belt. However, the rollers for supporting and driving the belt increase the number of parts and, therefore, the cost of the apparatus. In addition, the resulting belt unit is bulky to prevent the entire apparatus from being miniaturized.